Fix for foreign keys missing in generated ddl for all backends

0.9.0

Important

• Braking Fix: Version 0.8.0 introduced a bug that prevents generation of foreign_keys constraint in the database,
  both in alembic and during creation through sqlalchemy.engine, this is fixed now.
• THEREFORE IF YOU USE VERSION >=0.8.0 YOU ARE STRONGLY ADVISED TO UPDATE cause despite
  that most of the ormar functions are working your database CREATED with ormar (or ormar + alembic)
  does not have relations and suffer from perspective of performance and data integrity.
• If you were using ormar to connect to existing database your performance and integrity
  should be fine nevertheless you should update to reflect all future schema updates in your models.

Breaking

• Breaking: All foreign_keys and unique constraints now have a name so alembic
  can identify them in db and not depend on db
• Breaking: During model construction if Meta class of the Model does not
  include metadata or database now ModelDefinitionError will be raised instead of generic AttributeError.
• Breaking: encode/databases used for running the queries does not have a connection pool
  for sqlite backend, meaning that each querry is run with a new connection and there is no way to
  enable enforcing ForeignKeys constraints as those are by default turned off on every connection.
  This is changed in ormar since >=0.9.0 and by default each sqlite3 query has "PRAGMA foreign_keys=1;"
  run so now each sqlite3 connection by default enforces ForeignKey constraints including cascades.

Other

• Update api docs.
• Add tests for fk creation in db and for cascades in db